Make sure that you Freeze the RotoBrush before you try and render. This will lock the mask and speed up things dramatically.
What Rick said... freeze it. The issue you are facing is that the dynamic solution cannot be held in memory (in addition to the actual render stuff) and thus is recalculated every few frames. Freezing it will instead load it from the project file on disk and should be much faster.
Ok thanks guys, I'll try it and see what happens. I'll let you know the results. Thanks again
What's the source codec? If it's not WMV, you're transcoding as well as rendering. If your source is something kooky like HDV or H.264, you're decompressing, rendering, AND transcoding.